The location of the maximum methylmercury concentration at depth in the Pacific Ocean was the first evidence noted by the researchers pointing to the new methylation cycle. The graphic shows sampling depth on the left (in meters), and oxygen concentration on the right (in micromoles per kilogram of seawater [µmol/kg]) along a north-south latitudinal transect in the eastern North Pacific Ocean. The specific depth of maximal methylmercury concentration was consistently found at the ocean depth where the most rapid loss of oxygen was also observed. The process linking these two observations is microbial decomposition of "ocean rain", which is settling algae produced near the surface of the ocean. The decomposition process consumes oxygen from the water, but also leads to unintended methylmercury production.